1. Field of the Invention
This invention relates generally to lithographic processing. More particularly, this invention relates to an improved bearing arrangement for a reaction mass used for lithographic processing within a controlled environment.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. During lithography, a wafer is disposed on a wafer stage and held in place by a chuck. The chuck is typically a vacuum or electrostatic chuck capable of securely holding the wafer in place. The wafer is exposed to an image projected onto its surface by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the relevant art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of a reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized.
While using a step-and-scan technique generally assists in improving overall image sharpness, image distortions may occur in such systems due to movement of the entire system caused by the acceleration of the reticle stage or wafer stage. One way to correct this is by providing a counter balance (also referred herein as a reaction mass) to minimize the movement of the lithographic system upon acceleration of a stage. The reaction mass reacts to the force applied by the acceleration of a stage to prevent that force from affecting the overall lithography system during processing. In this way, reaction mass mechanisms eliminate stage-induced system vibration, which has the beneficial effect of increasing focus budgets and image contrast. Using a reaction mass mechanism allows rapid-motion scanning without transferring reaction forces to the machine baseframe, thus avoiding shaking the lithography system with every pass of the stage, which would disturb sensitive processing.
Typically, reaction mass mechanisms are guided by bearings or flexures. Flexures are thin vertical plates, attached at a protrusion at each end of a reaction mass such that when a stage coupled to the reaction mass accelerates, the reaction mass moves in the opposite direction as the stage, in a manner supported and guided by the flexures. The movement of the reaction mass in the opposite direction helps to stabilize the lithography system during processing. The ends of the flexures that are not coupled to the reaction mass are coupled to another entity, such as a baseframe. In this way, both ends of a flexure are constrained so that the flexure cannot rotate upon movement of the reaction mass. Flexures usually include one or more groove-like channels at each end for flexibility in supporting the reaction mass. The channels can be angular, rounded, or of any shape that will allow flexibility in the flexure.
An advantage of using flexures is that flexures can be used to guide one or more reaction masses in controlled environments such as purged gas mini-environments or high vacuum chambers. An advantage that the use of flexures has over the use of bearings is that flexures will not contaminate the environment as would, for example, the gas of a gas (or air) bearing or the lubricant of a roller bearing. Typically, flexures that are used in these controlled environments are flexures that are capable of accommodating a limited range of motion. By increasing the mass of the reaction mass, the required range of motion of the reaction mass is reduced to the point that flexures can be used. A reaction force is the product of the mass of the stage and the acceleration of the stage. Since the reaction force is also equal to the product of the mass of the reaction mass and the acceleration of the reaction mass, increasing the mass of the reaction mass reduces its acceleration, velocity and displacement.
However, the use of flexures presents a variety of problems in addition to extra-heavy reaction masses. For example, upon acceleration of a stage coupled to a reaction mass, the reaction mass will move in an arcuate path instead of the desired straight line due to the flexibility of the flexures. In other words, the flexures each shorten with a quadratic error. The effect of the quadratic error is an unbalanced up-and-down motion of the reaction mass. The arcuate motion caused by the quadratic error results in undesirable vertical reaction forces. The straying from straight line motion causes transverse forces to transfer to the baseframe of the system. Not only could this cause unwanted movements of the system during lithographic processing, but this also may cause a clearance problem between the bouncing reaction mass and a linear motor, if used to drive the stage. Complex flexure systems have been proposed, which in theory produce a straight line. However, in practice, the straight line motion is highly sensitive to manufacturing tolerances. For example, all flexures used would have to be exactly the same length, bend in exactly the same way, and be attached perfectly for a purely straight line motion to result.
Another shortcoming of flexure use is that the bending of flexures, alone, transfers some of the reaction force to the baseframe. Another shortcoming when using a heavy reaction mass is that it is difficult to achieve infinite fatigue life of the flexures. To achieve an infinite fatigue life, the flexures would have to be very long, which becomes difficult to package.
The use of bearings, on the other hand, provides a simpler arrangement that naturally produces substantially straight motion. One shortcoming of bearing use in general, however, is that a number of bearings are needed to guide the reaction mass (e.g., some are needed underneath the reaction mass, some are needed on the sides, etc.). With a split reaction mass stage, where at least two reaction masses are used, at least twice as many bearings are needed.
Although various types of bearings can be used (e.g., ball bearings, roller bearings, wheels, etc.), gas bearings are preferred in lithography systems because of good rectilinear motion. When using gas bearings, movement remains planar as long as the surface traveled over is fairly planar. Gas bearings do not present “lack of roundness” and “stick-slip” issues as one may have with wheel or ball bearings. The extremely low friction of gas bearings also conserves momentum, minimizing motor size. In addition, transmitted vibration is significantly reduced when using gas bearings because air is used instead of a solid object such as a ball. Potential contaminants, such as the lubricant in a ball or roller bearing are not present with gas bearings.
Although cylindrical gas bearings have been used with cylindrical rods as guideways inside high vacuum systems, their use is not favored for supporting the reaction masses of lithography systems. The main problem is that the cylindrical configuration is not well suited for supporting a heavy reaction mass. Large guide rod diameters are required for sufficient lift and to minimize guide rod deflection under the heavy load. Dynamically sealing against gas leakage into the vacuum chamber requires at least two pre-vacuum grooves in each cylindrical air bearing, which in turn demand additional vacuum pumps, resulting in an expensive system. The dynamic nature of the seal can result in some leakage of air bearing gas into the vacuum chamber, which increases the required size of the main vacuum pumps. Potential failure of the seal poses a high risk of catastrophic contamination within the controlled environment.
What is needed is a reaction mass system used in conjunction with linear stages that stabilizes a lithographic system during processing in a controlled environment, without the deficiencies associated with reaction mass systems described above.